1. Technical Field
The present invention relates generally to vehicle steering systems. In particular, the present invention relates to an improved drive wheel steering system for a lawnmower.
2. Related Art
Two basic types of steering systems are used for tractor-type power lawnmowers. The first type of steering system is a conventional vehicle steering system wherein a first set of wheels, mounted either at the front or the rear of the mower frame, are turned in unison through a steering wheel mechanism, while a second set of wheels are driven by a traction system. The traction system is powered by an engine to allow the mower to propel itself over the ground. The drive wheels may include a differential device to prevent wheel skid during turns.
The second type of steering system, which the present invention involves, is referred to as drive wheel steering, whereby two drive wheels are on fixed axles and do not turn about a vertical axis. A set of caster wheels, often mounted at the front of the mower frame, are not driven or steered, but are able to rotate about vertical axes during operation of the mower. Turning is achieved in these systems by rotating the two drive wheels at different speeds or in different directions for a very sharp turn, or even a zero turn radius wherein the mower actually turns about the mid-point of the drive wheel axis. In drive wheel steering systems, the mower becomes much more maneuverable than conventional vehicle steering, and slow mowing speeds are conducive to this improved maneuverability.
Hydraulically powered drive wheel systems typically include a separate variable displacement hydraulic pump for each of the two drive wheels. These variable displacement pumps are typically of the axial piston type which include a tiltable swash plate that can vary the pump discharge rate from a zero flow, also referred to as neutral, up to a maximum flow in a forward direction or up to a maximum flow in a reverse direction. This variable flow rate takes the place of a transmission and a clutch. While conventional steering is easily accommodated by a steering wheel through a mechanical linkage, drive wheel steering systems, wherein flow rates to the wheel motors control the turning, are more difficult to adapt to a conventional steering wheel.
Drive wheel steering type mowers have typically been controlled by a pair of side-by-side levers which move forward and aft from a neutral position to achieve steering. A primary disadvantage of this type of steering system is that in either an acceleration or deceleration condition, the momentum of the operator and his arms on the control levers have a tendency to apply a reverse input control to the system. For example, when the operator pulls the two control levers aft in attempting to decelerate the mower, the forward momentum of the driver tends to move the levers in the reverse direction. This problem can be particularly pronounced when operating a walk-behind lawnmower, because the operator has no stationary point of reference on the mower to use for stabilization. Various stabilizing bars have been proposed for attachment at the rear of walk-behind mowers that provide the operator with a surface on which to lean. However, the use of stabilizing bars generally tend to restrict the operator's freedom of hand movement behind the mower. Moreover, these stabilizer bars are of limited utility because the bars are intended merely as rest for the operators palms, rather than providing a positive gripping surface.
Other conventional drive wheel steering mowers have been controlled by a pair of side-by-side levers that each include a “pistol grip” type handle control. Using such a system, the operator controls the speed of the left and right drive wheels by manually manipulating the left and right pistol grip controls, respectively. The use of pistol grip controls, however, presents several problems. Repeatedly compressing and releasing the pistol grip controls during operation of the mower quickly proves tiresome, leaving the operator's hands sore and fatigued. Further, because the operator must keep a relatively loose grip on the levers in order to manipulate the pistol grips, mowers that incorporate pistol grip style steering systems are difficult to turn and keep steady. Moreover, it takes a significant amount of time to become comfortable and adept with the physical movements required to operate pistol grip style steering systems.
Drive wheel steering systems have also been devised that utilize a single lever for controlling both the speed and turning function of lawnmowers. However, such systems are generally complex, requiring one set of linkages to control the turning function and a second, separate set of linkages to control the speed of the mower. The turning function linkages of such systems are usually connected to the bottom of the steering column. This presents another disadvantage, particularly when used on walk-behind mowers. Connecting linkages to the bottom of the steering column requires that the steering column extend downward to a point close to the pumps. Thus, the steering column restricts the area underneath the steering lever in which the operator is permitted to move when walking behind the mower.
A need therefore exists for an improved drive wheel steering system for a lawnmower that overcomes the shortcomings found in the prior art.